Cells in a MONOS (metal-oxide-nitride-oxide-silicon) type nonvolatile semiconductor memory device have recently been advanced to downsizing.
During writing in the MONOS type nonvolatile semiconductor memory device, it is necessary to suppress leakage current from a charge storage insulator through a block insulator into a gate electrode. Further, during erasing, it is necessary to suppress electron injection from the gate electrode into the charge storage insulator.
During charge retention, it is necessary to suppress leakage of accumulated electrons from the charge storage insulator by a tunnel insulator and the block insulator.
However, as cells become finer in scale and higher in density, it is necessary to reduce a film thickness of the tunnel insulator or the block insulator for use in memory cells. Therefore, during writing or erasing, during repeated writing or erasing, or during charge retention, there arises a problem that desired device characteristics and reliability cannot be achieved.